It is well known in steel casting to employ a one-piece refractory stopper rod, which is moved vertically by the use of a lifting mechanism in order to vary the cross-sectional area of an outlet opening of the corresponding metallurgical vessel.
Those stopper rods have also been used to introduce an inert gas, such as argon, into the molten steel for removing non-metallic inclusions from the molten metal.
In all cases the stopper device must withstand hours submerged in molten metal. It must also be capable of enduring the harsh thermal shock encountered on the start-up of casting and any mechanical forces imposed to it.
Insofar many attempts have been made to improve the mechanical and thermal properties of such a stopper device and to improve its behaviour during use.
EP 0 358 535 B2 discloses a one-piece refractory stopper rod adapted to a lifting mechanism, comprising an elongated stopper rod body of a refractory material, which body being provided with a bore hole, having a longitudinal axis and extending from an upper surface of said body downwardly. Within said axial bore hole a metal bushing is inserted to threadably receive a threaded part of a metal rod, inserted in said refractory body for attachment to a corresponding lifting mechanism.
In a stopper rod for introducing gas into the melt it is important to provide a sealing between the refractory body and the metallic rod in order to prevent substantial loss of said gas and the infiltration of air.
To improve the required tightness it was proposed to place an annular gas tight gasket between the corresponding sealing surfaces. According to EP 1 135 227 B1 the axial bore hole of the body has an enlarged part that presents an annular sealing surface spaced away from the upper end of the body. A ring shaped graphite gasket is placed on said annular sealing surface and cooperates with a collar located on the rod.
This stopper design generates the seal in an axial manner, between like surfaces, with associated service risks of disruption of the seal by an increased expansion effects of the metallic rod compared to the surrounding ceramic body.
The same is true with a stopper design according to EP 0 358 535 B2.